systemd autopkgtest regression on arm64 and s390x on mantic

tests-in-lxd seems to fail to download the lxd image used for the test (or something similar), looking at the log:
...
Publishing instance: Image pack: 85% (5.51MB/s)

Instance published with fingerprint: 83b511be44395f2afc7d4d1ee0708cb18834c8b15cb208c9d218964ed8479114
6845s autopkgtest [09:11:14]: starting date and time: 2023-10-03 09:11:14+0000
6845s autopkgtest [09:11:14]: version 5.28ubuntu1
6845s autopkgtest [09:11:14]: host autopkgtest; command line: /usr/bin/autopkgtest -U -B . --test-name=unit-tests -- lxd autopkgtest/ubuntu/mantic/arm64
7063s <VirtSubproc>: failure: (down) ['mktemp', '--directory', '--tmpdir', 'autopkgtest.XXXXXX'] failed (exit status 1, stderr '')
7063s autopkgtest [09:14:52]: ERROR: testbed failure: unexpected eof from the testbed
7063s autopkgtest [09:14:52]: test tests-in-lxd: -----------------------]
7064s autopkgtest [09:14:53]: test tests-in-lxd: - - - - - - - - - - results - - - - - - - - - -
7064s tests-in-lxd FAIL non-zero exit status 1
2:40 PM
...

This introduces a regression that is blocking the promotion of new kernels in Mantic.

ProblemType: Bug
DistroRelease: Ubuntu 23.10
Package: systemd 253.5-1ubuntu6
ProcVersionSignature: Ubuntu 6.5.0-5.5.1-lowlatency 6.5.0
Uname: Linux 6.5.0-5-lowlatency x86_64
ApportVersion: 2.27.0-0ubuntu4
Architecture: amd64
CasperMD5CheckResult: pass
CurrentDesktop: ubuntu:GNOME
Date: Wed Oct 4 14:38:15 2023
InstallationDate: Installed on 2023-09-25 (9 days ago)
InstallationMedia: Ubuntu 23.10 "Mantic Minotaur" - Beta amd64 (20230924)
MachineType: {report['dmi.sys.vendor']} {report['dmi.product.name']}
ProcEnviron:
 LANG=en_US.UTF-8
 PATH=(custom, no user)
 SHELL=/bin/bash
 TERM=xterm-256color
 XDG_RUNTIME_DIR=<set>
ProcKernelCmdLine: BOOT_IMAGE=/vmlinuz-6.5.0-5-lowlatency root=/dev/mapper/ubuntu--vg-ubuntu--lv ro quiet splash vt.handoff=7
SourcePackage: systemd
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 09/01/2022
dmi.bios.release: 5.19
dmi.bios.vendor: American Megatrends International, LLC.
dmi.bios.version: 2.04
dmi.board.asset.tag: Default string
dmi.board.name: G1621-02
dmi.board.vendor: GPD
dmi.board.version: Default string
dmi.chassis.asset.tag: Default string
dmi.chassis.type: 10
dmi.chassis.vendor: Default string
dmi.chassis.version: Default string
dmi.ec.firmware.release: 2.3
dmi.modalias: dmi:bvnAmericanMegatrendsInternational,LLC.:bvr2.04:bd09/01/2022:br5.19:efr2.3:svnGPD:pnG1621-02:pvrDefaultstring:rvnGPD:rnG1621-02:rvrDefaultstring:cvnDefaultstring:ct10:cvrDefaultstring:skuDefaultstring:
dmi.product.family: Default string
dmi.product.name: G1621-02
dmi.product.sku: Default string
dmi.product.version: Default string
dmi.sys.vendor: GPD